Conventional solar cells convert light into electricity by exploiting the photovoltaic effect that exists at semiconductor junctions. In other words, the commercial solar cells absorb energy from visible light and converts excited charge carriers thereof to electric energy. At present, the main commercial solar cells are silicon-based solar cells. For a silicon-based solar cell, there are shortcomings in that high energy costs for material processing is required and many problems to be addressed such as environmental burdens and cost and material supply limitations are involved. For an amorphous silicon solar cell, there are also shortcomings in that energy conversion efficiency decreases when used for a long time due to deterioration in a short period.
Recently, many attempts have been undertaken to develop low-cost organic solar cells, whereby development of one particular type of solar cell which is a dye-sensitized solar cell (DSSC) is accelerated that is a class of thin film solar cells, is based on a semiconductor formed between a photo-sensitized anode and an electrolyte, and generally uses an organic dye to absorb incoming light to produce excited electrons.
The DSSC offers the prospect of a cheap and versatile technology for large scale production of solar cells. The dye-sensitized solar cell (DSSC) is formed by a combination of organic and inorganic components that could be produced at a low cost. The dye-sensitized solar cells have advantages over silicon-based solar cells in terms of simplified processing steps, low fabrication cost, transparency and pleochroism. The dye-sensitized solar cells can be fabricated from flexible substrates to function as cells of mobility and portability. Dye-sensitized solar cells have also the advantage to be lightweight.
The dye-sensitized solar cells have lower energy (photoelectric) conversion efficiency over that of the silicon-based solar cells such that a wide range of researches are briskly under way to enhance the energy conversion efficiency. In order to improve the energy conversion efficiency, extension of wave length up to infrared regions is being waged with great concern. It is known that the energy bandgap (eV) for use in solar cells must exceed 1.4 eV (electron volt).
The basic element of a DSSC is generally a TiO2 (titanium dioxide) nanoparticulate structure sensitized with dye molecules to form its core of a DSSC. The assembly of titanium dioxide nanoparticles is well connected to their neighbors. TiO2 is the preferred material for the nanoparticles since its surface is highly resistant to the continued electron transfer. However, TiO2 only absorbs a small fraction of the solar photons (those in the UV). The dye molecules attached to the semiconductor surface are used to harvest a great portion of the solar light.
The main dye molecules consist on one metal atom and a large organic structure that provides the required properties (wide absorption range, fast electron injection, and stability), such as ruthenium complexes. The dye is sensible to the visible light. The light creates and excitation in the dye that consists on a highly energetic electron, which is rapidly injected to the semiconductor (usually TiO2) nanoparticles. The nanoparticulate semiconductor functions as the transporter of light induced electrons towards the external contact, a transparent conductor that lies at the basis of the semiconductor (usually TiO2) film.
Meanwhile, phthalocyanine is an intensely colored macrocyclic compound that is widely used in dyeing. Phthalocyanines form coordination complexes with most elements of the periodic table. These complexes are also intensely colored and also are used as dyes. Approximately 25% of all artificial organic pigments are phthalocyanine derivatives.
Phthalocyanine (Pc) compound used as a dye in electrodes for solar cells has advantages such as high transmittivity relative to visible light, excellent selective absorption power in the near infrared region, high heat resistance, high weatherability and high thermotolerance, so that the phthalocyanine compound has a wide range of applications.
For instance, the use of zinc phthalocyanines in DSSCs has been disclosed by Md. Nazeeruddin et al., Angew. Chem. Int. Ed., 2007, 46, 373-376 and by M. Grätzel et al., Solar Energy Materials & Solar Cells, 2007, 91, 1611-1617.
However, there is still a need for dyes that could lead to an improvement of DSSCs, in particular to improved conversion efficiency. More particularly, there is still a need for dyes exhibiting a broad spectrum of adsorbed light (i.e. absorbing as much of the solar spectrum as possible), a high molar extinction coefficient, contributing to the long-term stability of the device and/or allowing an improved conversion efficiency.